1. Field of the Invention
The present invention relates to a method of reducing, or substantially preventing, ground disaster caused by a liquefaction phenomenon likely to occur in naturally stratified grounds composed of loose sand or sandy soil, piled grounds, filled-up grounds, and so on during earthquakes, and for restoration or reconstruction of disaster-stricken ground areas.
2. Description of the Prior Art
For Japan or a country where large cities are concentrated on seaside open fields made up of soft grounds, it is of vital importance to reconstruct soft grounds or provide for measures to fend off earthquake disasters when infrastructures such as water fronts, and slope fronts are developed, or put in good condition.
So far, the following countermeasures have been taken to prevent the aforesaid liquefaction.
(i) Easy-to-liquefy soil is dug out, and replaced by gravelly material of good water permeability.
(ii) Ground is compacted as by means of vibration rollers, vibroflotation methods, or sand compaction piles to increase the density and strength of loose sand or sandy soil.
(iii) By use of pile foundation, piles are driven through an easy-to-liquefy stratum or a stratum portion expected to liquefy into a stable sub-stratum.
(iv) Although depending on constructional, and environmental conditions, a piling structure is laid on ground or an underground water level is lowered to increase underground effective stress.
(v) When a filled-up ground is created, finely particulate soils are often used for reclaiming material usually by means of a sand pump. Such reclaiming material is substituted by difficult-to-liquefy soils made up of coarse soil particles.
(vi) Sand drains or gravel drains composed of coarse material are set up on ground expected to liquefy to thereby dissipate excess pore water produced during an earthquake.
Referring to an impregnating material used to obtain the first effect according to the present invention, as set forth in claim 1, the following standard mortar formulations may be used as standard grout composition although the inventive method is quite distinguishable over currently available methods.
TABLE 1 ______________________________________ Standard Mortar Formulations % by weight per 1,000 liters upon mixed Weight Ratio C:S:W Material 1:1:0.55 1:2:0.65 *1:3:0.75 Marginalia ______________________________________ Normal Portland 803.4 580.7 454.7 cement in kg Standard sand in kg 803.4 1,161.4 1,364.0 Dispersant in kg -- -- -- 0.25%/cement wt. Water in kg 441.9 377.5 341.0 ______________________________________ Note: The cement and standard sand used have a true specific gravity of 3.15 and 1.65, respectively.
Referring to an impregnating material composed of cement (C) and water (W) at a C to W ratio of 1:0.47, the following cement paste formulations are used as standard grout composition.
TABLE 2 ______________________________________ Cement Paste Formulations % by weight per 200 liters upon mixed Weight Ratio C:W Material 1:10 1:8 1:6 1:4 1:2 1:1 ______________________________________ Normal Port- 19.38 24.05 31.67 46.32 86.30 151.80 land cement in kg Water in kg 193.80 192.37 190.00 185.30 172.60 151.80 ______________________________________ Note: The cement used had a true specific gravity of 3.15.
Even upon built up on soft ground or filled-up ground not fully reconstructed by the means set forth in (i), (ii), (iii), (iv) and (v), ferro-concrete structures or skyscrapers are little, if any, hit by the liquefaction phenomenon caused by earthquakes, because pile foundations are often used for their foundation. If ground zones between the pile foundations or ground zones adjoining to the structures are liquefied, however, vital functions such as water pipes, gas pipes, and sewers, all called in Japan life lines, will be unavoidably broken or cut off.
Reference is made, on the other hand, to riparian structures, which are most likely to be hit by earthquakes although depending on their magnitude. If the foundation for banks alongside rivers is replaced by gravelly material or reinforced with drain material to prevent its liquefaction, there will then be a possibility that the banks are broken for reasons of flood waters, and unusual floods because water-permeable ground is formed beneath the levees. Riparian structures such as floodgates, and conduit pipes, too, are likely to suffer from damages such as subsidence, transversal flowing, and cracking between pile structures and a piling structure for the banks, because such phenomena as mentioned above arise.
Furthermore for naturally stratified ground with easy-to-liquefy loose sand or sandy soil placed thereon in the form of a thick layer or a wide area of created ground, it is required to reconstruct them by methods that enable the required minimum effect to be achieved in an economical manner.
A primary object of the present invention is to provide a solution to the aforesaid problems, and eliminate the problem set forth in (iv) above in view of constructional and environmental considerations.